It is desirable that a mobile radio system has high traffic handling capacity and a high degree of coverage. The traffic handling capacity of a mobile radio system is dependent, inter alia, upon the number of available radio channels and how effectively these channels may be utilised. It is known to arrange several base stations with small covering areas close to each other in a mobile radio system. Available radio channels can then be utilised in a more efficient way for handling peak traffic within a limited geographical area than if base stations with large covering areas are arranged far away from each other in the mobile radio system. Arranging several base stations close to each other can also increase the accessibility in a mobile radio system, particularly if the covering areas of the base stations are mutually overlapping. Two base station with such overlapping covering areas cannot, however, normally use the same radio channel for communication with different mobile stations.
When a mobile station moves from the area covered by one base station to the area covered by another base station in a mobile radio system, the responsibility for communication with the mobile must be transferred from one base station to another. Such transfer in conjunction with an established connection to the mobile station is usually called "handover".
Handover can be a critical function in known traditional mobile radio systems. There is a risk of the established connection being temporarily interrupted or completely lost in conjunction with the handover. There is usually required comparatively extensive signalling in the mobile radio system in conjunction with changing of radio channel at handover. The degree of coverage of a mobile radio system depends, inter alia, on the presence of radio shadows and how the covering areas of the individual base stations overlap each other. The possibility of establishing new connections and maintaining established connections to/from mobiles also depends on the presence of reflections and interference.
For increasing the degree of coverage in a mobile radio system, it is known to transmit substantially the same information to a mobile from two or more base transmitters and to receive information from a mobile at two or more base receivers. The publication "RF PERFORMANCE OF MACROSCOPIC DIVERSITY IN UNIVERSAL PORTABLE RADIO COMMUNICATIONS: FREQUENCY REUSE CONSIDERATIONS" by Richard C. Bernhardt, from IEEE International Conference on Communications, Jun. 22-25, 1986, Toronto, Canada describes different types of cells and placing of transmitters and the result of simulations of such a system. The publication "USER ACCESS IN PORTABLE RADIO SYSTEMS IN THE NOISE LIMITED ENVIRONMENT" by Richard C. Bernhardt, from ICC'87, Jun. 1987, Seattle, Wash., U.S.A., describes different types of cell and placing of transmitters as well as comparisons between different criteria and algorithms for the selection of base transmitter.
U.S. patent application Ser. No. 07/315,561 describes a mobile radio system and a method of transmitting message information where at least two bases at least partially simultaneously transmit substantially the same message information to a mobile. The object with this is, inter alia, to enable the degree of coverage to be made greater without the cells in a mobile radio system needing to be reduced, which gives greater freedom in the selection of cell pattern, and fewer handovers. The intention is that the mobile radio system shall be given better possibilities of establishing new connections and maintaining already established connections. According to the patent application, these objects are achieved by certain relationships between modulation of radio signals and parameters of adaptive equalisers in the mobiles and the distance between base transmitters which transmit the same message information.
In cellular mobile radio systems available channels may be allocated to cells and mobiles in various ways. In systems with fixed channel allocation a base for a particular cell may only use certain predetermined channels. Bases in adjacent cells use different channels. Bases in different cells sufficiently distant from each other may use same channels according to a channel re-use or allocation plan. In systems with adaptive channel allocation some or all channels are a common resource to all base stations, which means that some or all channels may be used by any base as long as the radio transmission conditions permit, i.e. certain carrier to interference or noise level.